an agent that needs several tools fails outright if the retriever misses any one of them, yet retrievers are mostly evaluated on single-tool queries. Measured across AppWorld and MCP-Bench, no untrained structure making at most one LLM call per query completes more than 10% of AppWorld queries at k=16; a set-level retriever trained on just 75–90 in-domain queries reaches .33 on AppWorld and .58 on MCP-Bench with zero LLM calls, while an LLM-navigated hierarchy needs a median of 31–38 calls and about five minutes per query to do worse.